The invention relates to hydraulic control systems useful for example, in controlling fixed displacement pumps wherein a plurality of pumps are provided for performing different functions, and especially to systems wherein some of the pumps in a circuit may be provided in pairs. The invention is especially useful in hydraulic systems used in heavy equipment such as earth-moving vehicles. An example is an excavator having an excavating bucket pivotally mounted on a stick which is in turn pivotally mounted on a boom. In such an excavator, a pair of pumps may be provided in a hydraulic circuit for operating hydraulic cylinders which move the stick and bucket and another pair of pumps may be provided in a second circuit having another pair of cylinders for operating the boom. In addition, in such equipment, the pumps may be used for other functions. For example, pumps in one of the circuits may also operate one track of the vehicle and the pumps in the other circuit may operate the other track. All pumps are typically driven by the vehicle engine through a common gear box.
In prior art systems, the pumps are generally driven by the operator at a constant speed. As is recognized, the input horsepower required of the prime mover which drives all of the pumps rises linearly with the pressure in the circuits and when the pressure rises substantially, as may occur when an obstruction is encountered during a digging operation, the torque requirements imposed on the prime mover may exceed the available torque. When this occurs, the diesel or gasoline engine used as a prime mover will stall. In the prior art, various arrangements are provided for unloading a pump in a circuit when an overload condition is encountered in that circuit which might cause the engine to stall or to cause damage to the equipment.
In the above-identified copending application an improvement in prior systems is disclosed in which positive displacement pumps driven by a common prime mover are arranged in operating circuits in a "crossover" unloading arrangement so that a pump in one circuit will be unloaded in response to pressure in either or both circuits. With the system disclosed in the prior copending application, one or more pumps in a plurality of circuits may be unloaded in response to pressure in one or more of the circuits so as to prevent the sum of the horsepower requirements from exceeding rated horsepower.
In certain applications, as for example in backhoes and various other kinds of excavators or earth-movers, it is desirable to allow the operator to exceed rated horsepower of the prime mover for short periods of time. In use of equipment not provided with pressure responsive unloading means, skilled operators recognize that they can exceed rated horsepower for short periods of time when encountering heavy or unusual loading conditions owing to the available flywheel or inertial energy of the prime mover. Thus, a skilled operator of manual equipment may rapidly break through an obstruction without stalling the prime mover whereas with the usual apparatus equipped with load sensing means, the operator will be unable to do so or may not be able to complete the job as rapidly since the pressure responsive mechanism will operate to prevent him from operating in a manner which results in rated horsepower being exceeded.